1. Field of the Invention
The present invention generally relates to mobile communication systems, such as air-to-ground communication systems, and more particularly, to a system which compensates for signal level changes and Doppler frequency shifts in the communication system without requiring precise a priori knowledge of the signal gains of the fixed or mobile components or frequency references. The invention enables mobile terminals to employ significantly lower cost frequency references while achieving performance comparable to implementations using much more precise references. In addition, the invention allows for dynamic compensation of other time, temperature, and unit-to-unit variations in gain and frequency which inevitably introduce errors into systems based on a priori measurements and knowledge of the gain and frequency profiles of the system components.
2. Description of the Prior Art
In an airborne communication system, the distance and rate of change of distance from the aircraft air terminal to a ground station are constantly varying. This introduces signal level changes and Doppler frequency shifts which must be compensated to minimize the interference potential to other services. In the prior art, high precision reference oscillators and a priori knowledge of the system gain factors are needed to ensure that the measurements made in the aircraft are correct. This approach is expensive to implement. What is needed is a less complex, and hence less expensive, alternative to the current compensation techniques used in the prior art.